This invention relates to an electromagnetic printing hammer device of a serial impact printer comprising a hammering element driven by an electromagnetic force to strike the back of each one of types arranged on a type wheel to print a symbol on a sheet wound on a platen.
In one type of serial impact printer known in the art, a petal type print wheel, such as a disc type print wheel, usually referred to as a daisy wheel, which comprises a multiplicy of types arranged on the circumference of an imaginary circle and each supported at a forward end of one of a multiplicity of types arranged on the circumference of an imaginary circle and each supported at a forward end of one of a multiplicity of spokes extending radially from the center of the imaginary circle, is connected to a shaft of a selection motor and angularly rotated by the action of the selection motor in accordance with a printing signal, to bring one of the types corresponding to the printing signal to a printing position in which the back of the type is struck by a hammering element to bring its type face into abutting engagement through an ink ribbon with a sheet wound on the platen, to perform printing of the desired character.
One type of printing hammer device is known for accomplishing the object of performing printing by using the daisy wheel referred to hereinabove in which an electromagnetic coil is wound on a yoke which is formed at opposite ends with a pair of electromagentic poles positioned opposite each other, and a member formed of a permanent magnet at least in one portion thereof is interposed between the electromagnetic poles in a gap defined therebetween for movement. In this type of printing hammer device, the member is moved by the magnetic attraction and magnetic repulsion that take place between the parmanent magnet of the member and the electromagnetic poles as a pulse current is passed to the coil, so that the member can serve as a printing hammer element.
FIG. 1a shows one example of this type of printing hammer device of the prior art. As shown, a coil C.sub.1 is wound on a yoke to provide an electromagnet having two poles a and b positioned opposite each other and defining a gap C therebetween in which a permanent magnet d is located between the two poles a and b to serve as a hammering element e. The hammering element e is supported for pivotal movement about a shaft f spaced apart from the gap C. The hammering element e moves in pivotal movement about the shaft f, so that the permanent magnet d also moves in circular movement in the same direction as the hammering element e. Because of this, the magnetism in the gap C and the magnetism of the permanent magnet d which repulse each other could not cross each other linearly, thereby making it possible to provide an effective thrust to the hammering element e. This would reduce the striking force with which printing is carried out by the action of the hammering element. Thus, to increase the striking force of the hammering element e, it would be necessary either to increase the value of the current passed to the coil C.sub.1 or to increase the magnetic force of the permanent magnet d.
To increase the value of a current passed to the coil C.sub.1 would be undesirable, because the coil and wires would have to have their capacities increased and power consumption would rise. This would make it necessary to increase the size of the electromagnetic printing hammer device, and means would have to be provided for dissipating heat which would increase in amount as power consumption rises.
Meanwhile, there are limits to the degree to which the magnetic force of the permanent magnet d can be increased even if a rare earth magnet were used.
The increase in power consumption noted hereinabove would be a disadvantage in the case of equipment using an electromagnetic printing hammer device. An increase in the size and weight of the electromagnetic printing hammer device would have the disadvantage that it runs counter to the tendency of the equipment using such hammer device becoming smaller in size and lighter in weight.
FIG. 1b shows a permanent magnet in which electromagnetic poles h and a yoke h' provided with a coil i are in staggered relation. In a magnet of the type in which the yoke h' is as shown, a leak magnetic flux of the coil i would exert influences on a permanent magnet j of the hammer section located in a magnetic field in addition to a thrust I, so that a force oriented in the direction of an arrow II would act thereon. Thus, a force that is wasted would be generated by the permanent magnet j in addition to the thrust I. Because of this, the hammer section of the permanent magnet shown in FIG. 1b has suffered the disadvantage that the thrust of the hammer section having the permanent magnet tends to be wasted.